1. Field of the Invention
This invention relates to a radar detector/security device for automobiles, and particularly to a radar detector/security device providing an existing radar detector installed in a motor vehicle with a security function utilizing the Doppler effect for the non-contact detection of a moving dielectric body such as a human being.
2. Prior Art Statement
It has become the fashion to attach various accessories and auxiliary electrical equipments to fourwheeled motor vehicles. Particularly in the United States of America, radar detectors for detecting the radar waves used in automobile speed traps and security devices known as car burglar alarms for preventing theft from an unattended car or even theft of the car itself are popular.
As the object of detection of these devices differs, they have of course been separate, independent products. An example of a security device for detecting illegal intrusion into the interior of a car is one in which when the user is to leave the car with the key in the ignition, he leaves a power switch on so that any subsequent opening of the car door is detected and a horn or other such warning means installed in the car is sounded.
The circuit block diagram of a typical commercial radar detector 10 for detecting radar waves is shown in FIG. 1. This radar detector 10 consists of a horn antenna structure 12 having a microwave antenna or a wave guide structure continuing on therefrom, and a radar wave detection circuit 11 which can be incorporated as a purely electrical circuit.
In the United States, the frequencies of radar waves (microwaves) used by police in vehicle speed traps generally belong to the X-band or K-band. These radar waves f.sub.R are detected by the horn antenna 13 of the horn antenna structure 12.
Behind the horn antenna 13 are connected a mixing chamber 14 and a first local oscillation chamber 15 in that order. In the first local oscillation chamber 15 is incorporated a negative-characteristic diode 16 formed of a Gunn diode, IMPATT diode or the like for outputting a first local oscillation frequency signal f.sub.L.
Incorporated in the mixing chamber 14 is a beat detection diode 17 such as a Schottky barrier diode or the like for detecting the beat frequency of the first local oscillation frequency signal f.sub.L and the radar wave f.sub.R entering the horn antenna 13.
With the above arrangement, when it is to be made possible to detect both the X-band and the K-band, a harmonics mixing system is used, as is well known. With the center frequency of the X-band being 10.525 GHz and that of the K-band 24.150 GHz, with respect to the center frequencies, both bands have an equal zone allowance of .+-.100 MHz. Thus, if 11.558 GHz is selected as the center frequency of the first local oscillation frequency signal f.sub.L generated by the negative-characteristic diode 16, and with respect to the X-band the beat frequency of the subject fundamental wave and the input radar wave f.sub.R is adopted, and with respect to the K-band the beat frequency of the second higher harmonic of the fundamental wave f.sub.L, i.e. 11.558.times.2=23.116 GHz, a first intermediate frequency signal f.sub.IF1 =1033.+-.100 MHz can be obtained from the output from the beat detection diode 17 via a first intermediate frequency signal amplifier 18 that is of equal value for both bands.
This is the principle of harmonics mixing. The first intermediate frequency signal f.sub.IF1 thus obtained is further beaten down by an adjustable and scannable second local oscillation frequency signal produced by a second local oscillator 20 at a second mixer 19, which is a solid-state electronic circuit, converting it to a fixed second intermediate frequency signal f.sub.IF2 which is in the order of several tens of megahertz, and is then applied to a signal detection circuit 22 via an amplifier 21.
The aforementioned radar detector 10 is of the type referred to as superheterodyne or double-conversion, and there is also a single-conversion type in which frequency conversion takes place once within the wave guide (in the chamber).
Although the above-described conventional radar detector has room for improvement with respect to details of ease of use and greater compactness, there are few problems with the basic principle.
However, it should be noted that external leakage of the first local oscillation frequency signal f.sub.L has to be kept to a minimum. While this also depends on the design, adjustment and suchlike of a driving circuit (driver) 29 that drives the diode 16 to provide the first local oscillation frequency signal f.sub.L, unless this is done successfully it becomes a source of electrical interference with other electrical equipment.
In particular, with the inherent object of a radar detector involving its operation while the vehicle is in motion, the generation of such interfering waves could set up interference and noise in on-board computers and other such important electrical circuitry, which could result in major problems. Among conventional products there have in fact been those which exhibit considerable leakage because they have not been well adjusted.
There have also been problems with the operating principles of conventional security devices. For example, some such devices were unable to detect an illegal intrusion unless a car door was opened, and were therefore unable to detect an intruder who gained entry to the car by breaking a window.
Also, conventional warning devices are arranged generally so that they are turned off when the door key is used to unlock the car door, so that the warning device can be shut off even if the door is illegally unlocked by means of a piece of wire or the like, and therefore will not sound a warning.